For an actor who has always delighted in head-to-toe transformation, playing the part of the rebounding superstar is not one that appeals. It doesn't suit him much, anyway; his stardom has always been predicated on the wild abandon of his metamorphoses.

"I don't watch movies, so I don't know what other people are doing because I don't care what other people are doing," he says. "I want to do what I want to do, and if it works, great, and if it doesn't... I can pump gas again."

Scott Cooper's "Black Mass" is an expansive look at the bonds of old-neighborhood loyalties that fostered the FBI's disastrous shielding of Bulger's Winter Hill Gang, which eradicated Boston's Italian mafia only to replace it with a murderous Irish-Catholic fiefdom.

Based on the book by Boston Globe reporters Dick Lehr and Gerard O'Neill, it's the first fact-based movie about the notorious crime boss and FBI informant (memorably the inspiration to Jack Nicholson's gangster in "The Departed") since the long-vanished Bulger was arrested in California in 2011. He was later sentenced to two life terms for, among other things, his involvement in 11 murders.

"Black Mass" is a richly populated ensemble, including Joel Edgerton, Benedict Cumberbatch, and Julianne Nicholson. But Depp's Bulger is the centerpiece of the film, and it has prompted predictions of an Oscar nomination for Depp. In a joint interview with Cooper, the director of "Crazy Heart" and "Out of the Furnace," the two discussed the challenges of portraying a folk-hero criminal who has, as Cooper said, "left a real emotional scar on the city of Boston."

"No disrespect to any victims or families of victims, but there was some element for me that was kind of glad that he got away," says Depp. "For 16 years he was on the lam and he wasn't causing any trouble. He was living his life. Good on him."

With blond hair slicked back and pale freckled skin, Depp's Bulger is harrowing in its sleazy ruthlessness and cold-blooded intimidation, bearing none of the whimsy that has accompanied some of the 52-year-old actor's recent films like "The Lone Ranger" and "Mortdecai." But Depp, who sympathetically played John Dillinger in Michael Mann's "Public Enemies," says he sought to find Bulger's humanity.

"You can't approach him as a gangster. You can't approach him as just innately evil because no one wakes up and shaves and goes, 'I'm... evil,'" says Depp. "He's a Catholic boy and kind of in a weird way a pillar of the community, very sensitive in a lot of ways."

Depp calls the first time he emerged from make-up during the Boston shoot and walked to set as "a frozen moment." Cooper, too, recalls vividly the chilled reactions by Bostonians, some of whom knew Bulger and perhaps weren't eager to see him resurrected on South Boston streets.

"Johnny takes risks as an actor that most movie stars won't even take because they're too fearful that they'll lose their audience, that they'll lose their status as a movie star," says Cooper, the director of the gritty blue-collar dramas "Crazy Heart" and "Out of the Furnace." ''The emotional and psychological transformation that I saw in the man who is sweet and gentle and kind and thoughtful into that? I don't even know where that comes from."

"I think they call it schizophrenia," retorts Depp.

Cooper and Depp both reached out to Bulger, now 86 and incarcerated in a Florida penitentiary, in hopes of sitting down with him, if only to soak up his personality and mannerisms. Depp says Bulger, not a fan of the book the film is based on, wrote a kind note declining.

In an email, Bulger's attorney, J.W. Carney, said: "Mr. Bulger considers the book 'Black Mass' to be largely a work of fiction, with made-up incidents and conversations that have little connection to the actual events."

But Cooper says the most important accuracy for him in a tale so tangled in perspective was in the larger meanings of Bulger's story.

"People don't come to narrative movies for the facts," he says. "They come to movies like this for psychological truth, for emotion and humanity. We did our best job telling our version of the story. But only these men know the truth."

The notion that "Black Mass" is any sort of "comeback" for Depp particularly riles Cooper. But Depp, whose recent string of box-office disappointments has been much chronicled, is merely bemused.

"The Mad Hatter is the Mad Hatter. What am I going to do, play him like Lee Majors?" Depp says. "Am I going to go in and play Tonto as the Native American with no humor or pride or war paint? Am I going to play Mortdecai straight? No, I'm going to have fun with it. If I have fun with, I believe the people will have fun with it. All of these films were... canned for hopefully none of my doing."

But after a run of fantasy, science-fiction, and comedy, Depp's Bulger is grimly grounded in reality, and a welcome reminder that Depp's talents of transformation work just as well in darkness as in light.

"Whatever they've done in their life – good, bad – they're still human at the end of the day, and you still have a responsibility. It's their life, and you have to pay great attention to that," Depp says. "Hopefully if (Bulger) is at all taken with the portrayal, he'll come out of nowhere and maybe send a letter."